On the Conflicting Pictures of Magnetism for the Frustrated Triangular Lattice Antiferromagnet CuFeO_2

摘要

The magnetic structures of the triangular lattice antiferromagnet CuFeO_2 below 14 K are described by an Ising model despite the fact that its high-spin Fe~(3+)(d~5)ions(S = 5/2,L = 0)cannot have a uniaxial magnetic moment.To resolve this puzzling picture of magnetism,we estimated the relative strengths of various spin-exchange interactions of CuFeO_2 by performing a spin dimer analysis and then determined the relative stabilities of a number of ordered spin states of CuFeO_2.Our calculations show that,in terms of a Heisenberg model,the noncollinear 120 deg spin arrangement predicted for a triangular lattice antiferromagnet is more stable than the collinear four-sublattice antiferromagnetic structure observed for CuFeO_2 below 11 K.To find a probable cause for stabilizing the collinear spin alignment along the c axis below 14 K,we considered the defect ions Fe~(2+)and Cu~(2+)of the CuFeO_2 lattice created by oxygen deficiency and oxygen excess,respectively.Our electronic structure analysis suggests that these defect ions generate uniaxial magnetic moments along the c axis and hence induce the surrounding Fe~(3+)ions to orient their moments along the c axis.